In a dual damascene step as a method of forming a multilayer wiring structure in a semiconductor device, there are formed, in an interlayer dielectric film, a via hole through which a wiring of an upper layer and a wiring of a lower layer are connected, and a trench (groove) in which a wiring of the upper layer is embedded. Copper as a wiring metal is embedded in these recesses.
In order to form the recesses such as via holes and trenches in the interlayer dielectric film, an etching step is performed by a plasma obtained from a process gas. Thereafter, a resist is ashed by a plasma obtained from an oxygen gas or a carbonic dioxide gas.
With a view to accelerating a signal transmission, formation of an interlayer dielectric film out of a material having a low relative dielectric constant has been studied. An SiCOH film is known as a representative low dielectric constant film.
However, in the above etching step and the above ashing step, the SiCOH film may be damaged by the plasma. In particular, in the ashing step, the SiCOH film may be seriously damaged because the SiCOH film is an organic film, while an oxygen gas is used in the ashing step. To be specific, electric properties of the SiCOH film may be considerably deteriorated.
As shown in FIG. 10A, this damage is caused when a connection between Si and a methyl group (CH3), which form the SiCOH film, is disconnected by the plasma of an oxygen gas, so that the methyl group, which has been disconnected from Si, is desorbed from the SiCOH film. On the other hand, the silicon (Si) from which the methyl group has been disconnected is prone to absorb moisture. Thus, the silicon takes therein moisture in an atmospheric air and in a process gas, or moisture generated by a reaction between oxygen in the process gas and hydrogen in the methyl group, resulting in further deterioration of the electric properties of the SiCOH film. Such a damage may invite various problems such as a broadening of a line width of a pattern after a wafer is washed, an increase in relative dielectric constant, an increase in leak current, and a deterioration in reliability caused by the moisture absorption.
Thus, a process for recovering the damage is performed according to the following manner. Namely, as shown in FIG. 10B, a silazane-containing gas including methyl groups is supplied to the SiCOH film so as to add, as shown in FIG. 10C, the methyl groups to the silicon from which another methyl group has been disconnected.
The ashing step is performed by using a parallel-plate type plasma processing apparatus under a condition that a damage given to the low dielectric constant film is restrained as much as possible. For example, a power to be applied to an upper electrode is set at about 300 W with respect to a 8-inch semiconductor wafer (referred to as “wafer” below). When the process gas is made plasma with this low application power, oxygen ions are mainly generated. The ashing step is performed by means of these oxygen ions.
However, as shown in FIG. 11, a surface part of a SiCOH film 100, in which molecules have become smaller because of the desorption of the methyl groups, provides a dense layer 101 of a higher density by an energy of the oxygen ions included in the plasma of the oxygen gas. Since this dense layer 101 acts as a solid obstacle, it is difficult for the silazane-containing gas having relatively larger molecules to be permeated into the SiCOH film 100. Thus, as shown in FIG. 11, only a superficial part of the dense layer 101 can be recovered by the recovery process.
JP2005-251837A (particularly, claim 1 and section 0037) describes that, when a resist pattern on an upper layer side of an organic low dielectric constant film is ashed, an application power to an upper electrode is not more than 0.81 W/cm2 per unit surface area of a wafer (an application power to the upper electrode with respect to an 8-inch wafer is not more than 255 W). However, this condition is an oxygen-ion rich condition, and thus the above problems cannot be solved.